1. Field of the Invention
The present invention relates to a method of detecting a light pulse reflected on an object, to a sensor and a device for implementing same making it possible to determine the distance separating an object from the sensor or the distance between two objects.
2. Description of the Related Art
The active light imaging principle consists in emitting a light pulse towards the scene or the object that is to be observed, detecting the light pulse after the latter has been reflected on the object, then displaying the detected signal on a display interface or processing the data for subsequent use. The detection is carried out using a detection device comprising a matrix of individual sensors each defining a pixel of the image. Examples of these are CCD (charge-coupled device) sensors and CMOS (complementary metal-oxide-semiconductor) sensors. Such a matrix these days comprises several millions of these individual sensors thus defining images comprising several megapixels.
Generally, two types of imaging are distinguished, namely 2D imaging and 3D imaging. 2D imaging is based on the emission of a light pulse which is reflected on a scene, followed by its detection by the matrix of individual sensors each of which collects a certain quantity of photons reflected by the observed object. Each sensor then transmits a signal proportional to the number of photons that it has received and this signal is displayed on each pixel of the image. There is thus obtained an indication on the reflectivity of the observed object relative to the scene surrounding it. Generally, the detection of the reflected light pulse is not carried out immediately after the emission of the light pulse but begins after a determined delay td, and is performed during a time period δt, called “integration time”.
The delay td determines the minimum observation distance Imin (because Imin=c*td/2, where c is the speed of light) and the integration time δt determines the maximum distance Imax (because Imax=c*(td+δt)/2). The closer the object is to the detection device, the more quickly the light pulse is reflected on the object. Thus, by controlling the delay td and the integration time δt, the user determines the objects that he can observe. This is illustrated in FIG. 1 in which a user (not represented) has an active imaging device A of the state of the art able to emit light pulses whose path is represented by the broken lines B. The light pulse is emitted at the time t0 by the device A towards the observed objects. The detection phase is delayed by a delay td relative to the moment of emission t0 and lasts for a time period δt. The part of the light pulse P1 which is reflected on the object O1 therefore returns to the device A before the detection phase begins, that is before the delay td. Consequently, the object O1 is not detected since it is located in front of the minimum observation distance Imin.
Between the time td and the time td+δt1, the individual sensors of the detection device store the photons of the part P2 of the light pulse reflected by the object O2. At the time td+δt1, each individual sensor transmits a signal proportional to the number of photons that it has received during the time interval δt1. This signal is transmitted to a data processing circuit then to an image display interface. This image is illustrated in FIG. 2 where it can be seen that the object O1 situated in front of the minimum observation distance is represented by a blurred silhouette, without contrast, with poorly-defined outline, whereas the object O2, whose signal has been picked up, is clear and contrasted. Finally, the part P3 of the light pulse reflected by the object O3 is not picked up because, given the distance from the object O3, it arrives after the integration time δt1. The object O3 therefore does not appear on the image. It should appear normally in black (absence of detection of photons), but for reasons of clarity in the figure, it has not been represented. The result displayed is a reflectance image which makes it possible to distinguish the objects from each other according to their capacity to return the light pulse. If the human eye can understand that the object O1 is located in front of the object O2, it is impossible to determine with precision by what distance the objects O1 and O2 are separated.
In order to access this information, the state of the art proposes reducing to the maximum the detection time δt and emitting a large number of light pulses with increasing delays td (see FIGS. 1, 3, 4 and 5), then combining the information from each image to “construct” the distance information. Thus, FIG. 3 illustrates that which is detected during the integration time δt1. The object O1 is not detected (illustrated in black), but the object O2 is detected. The image is coded as a distance image and represents the information supplied by all the individual sensors which have picked up a light signal during the integration time δt1. The distance D2 of the object O2 is calculated from the speed of propagation of light and the integration time δt1. All the objects situated behind the object O2 are not detected. They appear normally in black (absence of detection of photons), but for reasons of clarity in the figure, they have not been represented.
FIG. 4 illustrates that which is detected during the integration time δt2. The objects O1 and O2 are not detected (illustrated in black), but a first part O3a of the object O3 is perfectly detected. The image is coded as a distance image and represents the information supplied by all the individual sensors which have picked up a light signal during the integration time δt2. The first part O3a of the object O3 is then situated at the distance D3a. The part O3b of the object O3 situated behind the part O3a is not detected but has not been represented.
Finally, FIG. 5 illustrates that which is detected during the integration time δt3. The objects O1, O2 and O3a are not detected (illustrated in black), but the second part O3b of the object O3 is detected. The image is coded as a distance image and represents the information supplied by all the individual sensors which have picked up a light signal during the integration time δt3. The second part O3b of the object O3 is then situated at the distance D3b. 
Then, all the data is compiled so as to produce, from the images of FIGS. 3, 4 and 5, an artificial image, illustrated in FIG. 6, which represents the indication supplied by the individual sensors which have picked up a light signal during the integration times δt1, δt2 and δt3. This method is lengthy and complex because it entails processing a large quantity of data. It is costly in energy because a large number of light pulses is needed (in the example illustrated, three pulses are needed; in reality, their number is very much higher). Also, the spatial resolution of the 3D construction from all the 2D images is determined by the time accuracy of the delay offset td and the duration of the integration time δt. Finally, this method is sensitive to the movements of the objects during the succession of light pulses, so that the artificially constructed image is not always accurate.
Other methods of the state of the art for producing 3D imaging are available but to obtain a good resolution in terms of reflectivity and depth, the two indications are processed simultaneously, which requires fast and complex electronic circuits which can limit the resolution in depth and their lateral resolution by imposing a pixel pitch that is great enough to incorporate all the electronic detection compounds in the pixel.
There are also active imaging systems which use gain sensors which can be adjusted according to the distance of the objects to be observed or the power of the light pulses sent. Thus, when the energy loss is great (long observation distances and/or low energy light source), the gain of the sensors is set to the maximum in order to obtain a high sensor sensitivity. In these devices, the gain is an adjustment parameter of the device in the same way as the detection delay td and the duration of the integration time δt. However, once set, the gain does not vary during the integration time δt. Such a system is described in the article “a low noise, laser-gated imaging system for long range target identification” by Ian Baker, Stuart Duncan and Jeremy Copley, published in the review Proceedings of SPIE, volume 5406, pages 133-144, in August 2004. This system also uses a succession of laser pulses that have to be processed before display.